Sheathed-element glow plug

ABSTRACT

A sheathed-element glow plug to be mounted in a combustion chamber is proposed, a rod-shaped heating element being situated in a concentric bore hole of the housing. The heating element has a first current-carrying layer, a second current-carrying layer, and an insulating layer, the insulating layer separating the first current-carrying layer and the second current-carrying layer. The first current-carrying layer and the second current-carrying layer being connected at the end of the heating element on the combustion chamber side by a conducting-layer crosspiece. The first current-carrying layer and the second current-carrying layer are different lengths, the cross section of the first current-carrying layer in a first section at the end of the heating element away from the combustion chamber being greater than the cross section of its remaining length, and the second current-carrying layer not extending into the first section.

FIELD OF THE INVENTION

The present invention relates to a sheathed-element glow plug to bemounted in a combustion chamber.

BACKGROUND INFORMATION

Sheathed-element glow plugs, having a metallic housing, to be mounted ina combustion chamber are already known. A rod-shaped heating element issituated in a concentric bore hole of the known sheathed-element glowplug, the heating element having a first current-carrying layer and asecond current-carrying layer, the cross sections of the first and thesecond current-carrying layer being connected on the end of the heatingelement on the combustion chamber side via a conducting-layercrosspiece. In this context, the first and the second current-carryinglayers are separated by an insulating layer. Furthermore,sheathed-element glow plugs are known whose current-carrying layers varyin length.

SUMMARY OF THE INVENTION

In contrast, the sheathed-element glow plug of the present invention hasthe advantage that there is no danger of a short circuit at the end ofthe heating element away from the combustion chamber. A furtheradvantage is that the contact surface between the first current-carryinglayer and the contact element situated at the end of the heating elementaway from the combustion chamber is enlarged. The contact resistance isconsequently reduced, thereby resulting in the contact point heating upless. Therefore, the danger of the contact material between the heatingelement and the contact element being thermally destroyed is decreased.In addition, it is advantageous that the rod-shaped heating element doesnot need to be adjusted to remove an insulating layer situated on therod-shaped heating element in the region in which the current supply isto be contacted.

It is particularly advantageous to also design the insulating layer tobe asymmetrical, so that the danger of a short circuit due to damage ora porousness of an insulating layer deposited on the heating element isalso decreased in this instance. In this context, it is advantageous toexpand the region in which the insulating layer is asymmetrically formedin the direction of the combustion chamber over the collar of theheating element, since a summation of form-dependent andmaterial-dependent stress concentration is prevented in this manner. Inaddition, it is advantageous to design the heating element such that anadvantageously half-shell-shaped insulating layer made of anelectrically insulating, ceramic material is deposited in the region inwhich the first current-carrying layer extends into the housing, theinsulating layer between first and second current-carrying layer beingmade of the same material. As a result, the manufacturing process issimplified and, thus, more cost-effective. In order to be able todispense with additional insulation, it is advantageous to design theinsulating layer such that it extends beyond the end of the housing onthe combustion chamber side. Furthermore, it is advantageous to providea stepped lug at the end of the heating element away from the combustionchamber, so that an adapter sleeve situated on the end of the heatingelement away from the combustion chamber and the contact element areable to be easily positioned.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a longitudinal section of a sheathed-elementglow plug according to the present invention.

FIG. 2 schematically shows a longitudinal section of one exemplaryembodiment for a heating element of a sheathed-element glow plugaccording to the present invention.

FIG. 3 schematically shows a longitudinal section of another exemplaryembodiment for a heating element of a sheathed-element glow plugaccording to the present invention.

FIG. 4 schematically shows a longitudinal section of another exemplaryembodiment for a heating element of a sheathed-element glow plugaccording to the present invention.

FIG. 5 schematically shows a longitudinal section of the end of asheathed-element glow plug of the present invention on the combustionchamber side.

DETAILED DESCRIPTION

The longitudinal section of a sheathed-element glow plug of presentinvention is schematically shown in FIG. 1. The sheathed-element glowplug has a housing 3, which is preferably made of a metallic material, aheating element being situated in the housing's concentric, continuousbore hole, on the end on the combustion chamber side. The heatingelement includes a first current-carrying layer 11, a secondcurrent-carrying layer 12, and an intermediary insulating layer 15.First current-carrying layer 11 and second current-carrying layer 12 areconnected by a conducting-layer crosspiece 13 at the end of the heatingelement on the combustion chamber side. The described configuration offirst current-carrying layer 11, second current-carrying layer 12, andconducting-layer crosspiece 13 results in a U-shaped configuration ofthe current-carrying layers. First current-carrying layer 11 iselectrically contacted on the end of the heating element away from thecombustion chamber to a contact element 31, which is preferably designedas a graphite pellet or another flexible and conductive element (e.g. ametal spring). Contact element 31 and the end of terminal stud 35 on thecombustion chamber side, the terminal stud being situated on the end ofcontact element 31 away from the combustion chamber, are situated in afirst adapter sleeve 33, first adapter sleeve 33 being the shape of ahollow cylinder and being made of an electrically insulating material.Terminal stud 35 extends to the end of the sheathed-element glow plugaway from the combustion chamber and runs in the inner, concentric borehole of housing 3. In this context, additional elements (second adaptersleeve 37 and metal ring 39), which are the shape of a hollow cylinderand through which terminal stud 35 runs, are situated in this bore hole.A contact plug 40 representing the connection to the glow plug switchingcircuit is placed on terminal stud 35 on the end away from thecombustion chamber. A sealing ring 41, which seals the inside of thehousing of the sheathed-element glow plug from the external space, issituated between housing 3 and contact plug 40. This sealing ring 41 isalso in the shape of a hollow cylinder.

The second current-carrying layer is electrically contacted via a regionin which electrically insulating layer 16, which surrounds the end ofthe heating element away from the combustion chamber, is removed, andalso via sealant 5 to housing 3. Sealant 5 is situated around the end ofthe heating element away from the combustion chamber in the shape of aring and seals the inside of the housing in the direction of thecombustion chamber. In a preferred exemplary embodiment, a contact layer17 may also be deposited in the region in which the secondcurrent-carrying layer is to be contacted by sealant 5. Furthermore, acontact layer 17 on the end of the heating element away from thecombustion chamber may also produce the contact between firstcurrent-carrying layer 11 and contact element 31.

The construction of the heating element is to be described in moredetail on the basis of FIG. 2. FIG. 2 schematically shows thelongitudinal section of a heating element of a sheathed-element glowplug according to the present invention. Identical reference numeralsused in this and in the following figures in reference to FIG. 1designate identical elements. Therefore, this will not be discussedagain in detail. FIG. 2 shows that, on the end of the heating elementaway from the combustion chamber, in a first section 21, firstcurrent-carrying layer 11 has a cross section that is enlarged withrespect to the cross section of the remaining length of firstcurrent-carrying layer 11. As such, the longitudinal section of firstcurrent-carrying layer 11 has an asymmetrical L-shaped design. Theregions of first section 21 on the end of the heating element away fromthe combustion chamber that are not filled by first current-carryinglayer 11 are filled by insulating layer 15. Second current-carryinglayer 12 does not protrude into this first segment 21 of the heatingelement.

In a preferred exemplary embodiment, first current-carrying layer 11 isso significantly enlarged in first section 21 that the cross section offirst current-carrying layer 11 in this section corresponds to the crosssection of the heating element. The design of this exemplary embodimentmay also be seen in FIG. 1.

Widening the cross section of first current-carrying layer 11 ensures anenlarged contact area between first current-carrying layer 11 andcontact element 31, which is situated at the end of the heating elementaway from the combustion chamber. This increase in the contact arearesults in a decrease in the contact-resistance and, thus, in a lesssignificant heating of this region.

In the direction of the combustion chamber, a second section 22, inwhich the cross section of insulating layer 15 is asymmetricallyenlarged with respect to the cross section of its remaining length,i.e., the cross section in the direction of the combustion chamber,borders first section 21 of the heating element. Second current-carryinglayer 12 also does not extend into this second section 22. In thiscontext, the end of second section 22 on the side of the combustionchamber may be selected such that it is a component of theheating-element collar (see FIG. 1) or of the heating-element shaft (seeFIG. 2) or is situated exactly at junction 19 between theheating-element collar and the heating-element shaft. In this context,the region of the heating element having the greatest cross section atthe end away from the combustion chamber is referred to as theheating-element collar. The region of the heating element bordering theheating-element shaft in the direction of the combustion chamber and notbelonging to the heating-element collar is referred to as theheating-element shaft. Preferably, the end of second section 22 is notsituated such that it is precisely at junction 19 between theheating-element shaft and heating-element collar, since an additionalstress concentration due to the material junction at the particularlystressed point of the junction between the heating-element shaft and theheating-element collar is prevented in this manner. The proposedformation of insulating layer 15 effectively prevents a short circuitbetween first current-carrying layer 11 and second current-carryinglayer 12 at the end of the heating element away from the combustionchamber.

As shown in FIG. 2, an insulating layer 16, which is preferably designedas a glass coating, is situated at the end of the heating element awayfrom the combustion chamber, in the region of the shell. In region 17,in which second current-carrying layer 12 is in electrical contact withsealant 5, this insulating layer 16 is either interrupted or a contactlayer 17, which improves the contact between second current-carryinglayer 12 and sealant 5, is formed. This contact layer 17 may preferablybe designed as a metallic layer.

FIG. 3 schematically shows a longitudinal cross section of a furtherexemplary embodiment of a heating element of a sheathed-element glowplug of the present invention. This heating element does not have aninsulating layer 16, which completely surrounds the end of the heatingelement on the combustion chamber side, but only an insulating layer 18in the region in which the first current-carrying layer would be incontact with housing 3 without insulating layer 18. Insulating layer 18is preferably shaped like a half-shell. Section 23, in which insulatinglayer 18 is deposited, is referred to as third section 23 in thefollowing. In this context, it is advantageous when insulating layer 18extends from the end of the heating element away from the combustionchamber over the edge of the housing. Thus, a short circuit betweenfirst current-carrying layer 11 and housing 3 is effectively preventedas a result of the thickness of insulating layer 18 being up to several100μm.

In a particularly preferred exemplary embodiment, insulating layer 18 ismade from the same material as insulating layer 15. Consequently, amanufacturing process including the manufacture of a laminate ofelectrically insulating and electrically conductive ceramic layers ispossible in a particularly cost-effective manner, since all layers areable to produced using the same systems and devices. A process stepinvolving depositing a chemically different layer is consequentlyeliminated.

FIG. 4 schematically shows a further exemplary embodiment of a heatingelement of a sheathed-element glow plug according to the presentinvention. The heating element has on the end away from the combustionchamber a stepped lug 11′, which connects to first current-carryinglayer 11 and is made of the same material as first current-carryinglayer 11. This stepped lug is used to precisely place contacting element31 and first adapter sleeve 33, as also shown in FIG. 1.

FIG. 5 schematically shows a longitudinal cross section of a furtherexemplary embodiment of a sheathed-element glow plug according to thepresent invention. In this context, the Figure is limited to the end ofthe sheathed-element glow plug on the combustion chamber side. Thisdrawing is to be used to show again that insulating layer 18 or thirdsection 23 extends from the end of the heating element away from thecombustion chamber and adjacent to the housing beyond the edge ofhousing 3 on the side of the combustion chamber. As already described inlight of FIG. 1, a first adapter sleeve 33 and a contact element 31connect to the end of the heating element away from the combustionchamber. In a preferred exemplary embodiment, the end face of lug 11′ ofsecond insulating layer 11 facing away from the combustion chamber maybe provided with a contact layer 17, which improves the contact betweenfirst insulating layer 11 and contact element 31.

In all of the exemplary embodiments, first current-carrying layer 11,second current-carrying layer 12, and conducting-layer crosspiece 13 aremade of electrically conductive ceramic material. Insulating layer 15 ismade of electrically insulating material. The ceramic, electricallyconductive and electrically insulating materials are preferably ceramiccomposite structures including at least two of the compounds Al₂O₃,MoSi₂, Si₃N₄, and Y₂O₃. These composite structures are able to beobtained using a one-step or multi-step sintering process. The specificresistivity of the layers may preferably be determined by the MoSi₂content and/or the particle size of the MoSi₂. The MoSi₂ content offirst and second current-carrying layers 11, 12 and of conducting-layercrosspiece 13 is preferably greater than the MoSi₂ content of insulatinglayer 15.

In a further exemplary embodiment, first and second current-carryinglayers 11, 12, conducting-layer crosspiece 13, and insulating layer 15are made of a composite precursor ceramic having different proportionsof fillers. In this context, the matrix of this material is made ofpolysiloxanes, polysilsequioxanes, polysilanes, or polysilazanes thatmay be doped with boron or aluminum and are produced by pyrolysis. Atleast one of the compounds Al₂O₃, MoSi₂, and SiC forms the filler forthe individual layers. Analogously to the abovementioned compositestructure, the MoSi₂ content and/or the particle size of the MoSi₂ maypreferably determine the specific resistivity of the layers. Preferably,the MoSi₂ content of first and second current-carrying layers 11, 12 andof conducting-layer crosspiece 13 is greater than the MoSi₂ content ofinsulating layer 15.

The compositions of insulating layer 15, first and secondcurrent-carrying layers 11, 12, and conducting-layer crosspiece 13 areselected in the abovementioned exemplary embodiments such that theirthermal expansion coefficients and the shrinkage of the individual leadlayers, conducting-layer crosspiece layers, and insulating layersoccurring during the sintering or pyrolysis process are equal, so thatthere are no cracks in the sheathed-element glow plug.

What is claimed is:
 1. A sheathed-element glow plug to be mounted in acombustion chamber, comprising: a housing; a rod-shaped heating elementsituated in a concentric bore hole of the housing, the heating elementincluding a first current-carrying layer and a second current-carryinglayer, wherein: the first current-carrying layer has a length that isdifferent than a length of the second current-carrying layer; aconducting-layer crosspiece, the first current-carrying layer and thesecond current-carrying layer being connected to an end of the heatingelement on a combustion chamber side via the conducting-layercrosspiece; a first insulating layer for separating the firstcurrent-carrying layer from the second current-carrying layer, wherein:a cross section of the first current-carrying layer in a first sectionat an end of the heating element away from the combustion chamber isgreater than a cross section of a remaining length of the firstcurrent-carrying layer; and the second current-carrying layer does notextend into the first section; an external second insulating layersurrounding the first current-carrying layer over a length of a thirdsection of the heating element.
 2. The sheathed-element glow plugaccording to claim 1, wherein: the cross section of the firstcurrent-carrying layer in the first section corresponds to a crosssection of the heating element.
 3. The sheathed-element glow plugaccording to claim 1, wherein: the third section of the heating elementextends from the end of the heating element away from the combustionchamber beyond an end of the housing away from the combustion chamber.4. The sheathed-element glow plug according to claim 1, wherein: thefirst current-carrying layer, the second current-carrying layer, and theconducting-layer crosspiece are made of an electrically conductingceramic material, and the first insulating layer and the external secondinsulating layer are made of an electrically insulating ceramicmaterial.
 5. The sheathed-element glow plug according to claim 4,wherein: the first insulating layer and the external second insulatinglayer are made of the same electrically insulating ceramic material. 6.A sheathed-element glow plug to be mounted in a combustion chamber,comprising: a housing; a rod-shaped heating element situated in aconcentric bore hole of the housing, the heating element including afirst current-carrying layer and a second current-carrying layer,wherein: the first current-carrying layer has a length that is differentthan a length of the second current-carrying layer; a conducting-layercrosspiece, the first current-carrying layer and the secondcurrent-carrying layer being connected to an end of the heating elementon a combustion chamber side via the conducting-layer crosspiece; and afirst insulating layer for separating the first current-carrying layerfrom the second current-carrying layer, wherein: a cross section of thefirst current-carrying layer in a first section at an end of the heatingelement away from the combustion chamber is greater than a cross sectionof a remaining length of the first current-carrying layer, the secondcurrent-carrying layer does not extend into the first section; the crosssection of the first current-carrying layer in the first sectioncorresponds to a cross section of the heating element; a cross sectionof the first insulating layer in a second section is greater than across section of a remaining length of the first insulating layer; thesecond section borders the first section in a direction of thecombustion chamber; and the second current-carrying layer does notextend into the second section.
 7. The sheathed-element glow plugaccording to claim 6, wherein: an end of the second section of theheating element on the combustion chamber side is not situated at ajunction between a heating-element collar and a heating-element shaft.8. A sheathed-element glow plug to be mounted in a combustion chamber,comprising: a housing; a rod-shaped heating element situated in aconcentric bore hole of the housing, the heating element including afirst current-carrying layer and a second current-carrying layer,wherein: the first current-carrying layer has a length that is differentthan a length of the second current-carrying layer; a conducting-layercrosspiece, the first current-carrying layer and the secondcurrent-carrying layer being connected to an end of the heating elementon a combustion chamber side via the conducting-layer crosspiece; and afirst insulating layer for separating the first current-carrying layerfrom the second current-carrying layer, wherein: a cross section of thefirst current-carrying layer in a first section at an end of the heatingelement away from the combustion chamber is greater than a cross sectionof a remaining length of the first current-carrying layer; the secondcurrent-carrying layer does not extend into the first section; and thefirst current-carrying layer includes a stepped lug at an end of thefirst section of the heating element away from the combustion chamber.9. The sheathed-element glow plug according to claim 8, wherein: thecross section of the first current-carrying layer in the first sectioncorresponds to a cross section of the heating element.